Organic electroluminescent (EL) devices are generally composed of three layers of organic molecules sandwiched between transparent and metallic electrodes, the three layers including an electron transporting layer, an emissive layer and a hole transporting layer. The EL efficiency is limited, in part, by the fluorescence efficiency of the emitter material. For example, the EL device based on a well-known organic emitter, 8-hydroxyquinoline aluminum (Alq), has a quantum efficiency of 1% (photon/electron) (C. W. Tang et al, J. Appl. Phys. 1989, 65, 3610). It has also been demonstrated that the performance of an organic EL device such as fluorescence efficiency and intensity was enhanced by about 3-4 times by doping Alq with a dye molecule of higher fluorescence efficiency. In the same prior art, light emission from 510 to 650 nm, that is, from green to orange-red, was achieved by doping Alq with green dye coumarin 540 and orange-red dye DCM1 respectively. However, the short wavelength light, namely, blue emission can not be attained with green fluorescent Alq as a host emitter. In a later patent (Vanslyke et al U.S. Pat. Nos. 4,539,507; 5,150,006), blue emission was achieved by doping a greenish-blue emissive Alq derivative, with a blue dye such as perylene.
These dye molecules used as guest dopants in organic EL devices in the prior art are classified as pure organic molecules and have the tendency to undergo recrystallization in the solid state. On the contrary, Alq and its derivative as host emitter belong to a different class of compounds, namely, organometallic molecules. Since the guest dopants and host emitter are two different class of materials, they have different intermolecular interaction, making them less compatible with each other. Under stressful operation conditions, the guest dopant and the host emitter have the possibility to undergo phase-separation, which may result in premature device failure in the long run.
Although Alq and its derivatives are thermally stable up to 400.degree. C., most of the dye dopants have either melted or decomposed below 280.degree. C. Thus, the thermal stability of a dye-doped organic EL device is most likely limited by the thermal stability of the individual dye molecule.
It has been established in the prior art that the optimal doping concentration of the guest dye in host emitter is about 0.5% in an organic EL device. At slightly higher doping levels, the phenomenon of concentration quench starts to occur, which actually lowers the fluorescent efficiency. Therefore having a firm control on the doping concentration is critical to the device performance. While Alq and its derivative, the host emitter, sublime at about 300.degree. C. under 5.times.10.sup.-6 torr vacuum, most of the dye dopants sublime below 200.degree. C. As a result, the host emitter and the dye dopants generally have to be sublimed in two separate sources simultaneously. Since the host emitter normally has a thickness ranging from 200 to 400 .ANG., the dye dopants need to have a cumulative thickness of 1-2 .ANG. in order to achieve an optimal 0.5% doping concentration level. The host emitter usually is sublimed at a rate of 5 .ANG./second. To achieve a homogeneously doped emitting layer, the dye dopants have to be sublimed at a rate of 0.025 .ANG./second. which, if not impossible to achieve, will be hardly reproducible in a manufacture environment.
It is a purpose of this invention to provide a class of new organometallic complexes with built-in dyes for use in light emitting devices.
It is another purpose of this invention to provide preparation methods for the disclosed organometallic complexes with built-in dyes for use in light emitting devices.
It is a still another purpose of this invention to provide a class of new organometallic complexes with built-in dyes as guest dopants for use in light emitting devices.
It is yet another purpose of the present invention to provide a class of new organometallic complexes with built-in dyes for use in light emitting devices with greater thermal stability.
It is a further purpose of this invention to provide a class of new organometallic complexes with built-in dyes as guest dopants that can be co-deposited with a host emitter for use in light emitting devices.